Somatic and Special senses

Sensory Transduction →

Conversion of physical, chemical, or light stimulus energy into a change in membrane potential of a sensory receptor.

Stimulus Response Pathway

→ Stimulus opens Na⁺ channels → triggers action potential (nerve impulse).

Stimulus Types

→ Light, mechanical force, or chemicals.

Receptor Function

→ Initiates nerve impulses to the CNS.

Free Nerve Endings and Corpuscles

→ Types of sensory receptors (free nerve endings and connective tissue corpuscles).

Specialized Receptor Cells

→ Receptors that detect specific types of stimuli such as light, chemicals, or temperature.

Photoreceptors

→ Respond to light.

Chemoreceptors

→ Respond to taste, smell, oxygen, carbon dioxide, and pH.

Nociceptors

→ Pain receptors; protective function; do not adapt rapidly; respond to numerous stimuli; found in most tissues.

Thermoreceptors

→ Stimulated by hot and cold temperatures.

Touch Receptors

→ Free nerve endings, hair follicle receptors, Meissner’s corpuscles.

Pressure Receptors

→ Pacinian corpuscles, proprioceptors, baroreceptors.

Meissner’s Corpuscles

→ Touch receptors found near the epidermis.

Pacinian Corpuscles

→ Deep pressure receptors located toward the subcutaneous layer; have lamellar layers surrounding a neuron.

Proprioceptors

→ Found in joints, ligaments, and tendons; detect stretching, pressure, and pain; integrated with cerebellum to distinguish body position.

Baroreceptors

→ Found in walls of blood vessels, digestive, reproductive, and urinary tracts; detect pressure changes.

Sensory Epithelia

→ Consist of receptor cells and supporting cells.

Lamina Propria

→ Connective tissue layer supporting sensory epithelia.

Supporting Cells

→ Epithelial cells that support receptor cells.

Basal Cells

→ Stem cells that replace receptor cells.

Receptor Cells

→ Sensory neurons that detect stimuli.

Sensory Transduction (Special Senses)

→ Stimulus binds receptor → opens Na⁺ channels → initiates action potential → nerve impulse.

Stimulus Types for Special Senses

→ Chemical, light, heat, cold, pain, or mechanical force.

Vision

→ Sense of sight.

Visual Accessory Organs

→ Structures associated with protection and movement of the eye.

Conjunctiva

→ Mucous membrane covering the front of the eye and lining the eyelids; inflammation causes conjunctivitis.

Lacrimal Apparatus

→ Secretes enzyme lysozyme; reduces bacterial infections.

Extrinsic Eye Muscles →

• Superior Rectus: moves eye up

• Inferior Rectus: moves eye down

• Medial Rectus: moves eye medially

• Lateral Rectus: moves eye laterally

• Superior Oblique: rotates eye counterclockwise

• Inferior Oblique: rotates eye clockwise

Anterior Cavity (Eye)

→ Contains aqueous humor; maintains shape and refracts light.

Posterior Cavity (Eye)

→ Contains vitreous humor; supports and maintains intraocular pressure.

Three Layers of the Eye →

1. Fibrous Tunic (Sclera)

2. Vascular Tunic (Choroid)

3. Nervous Tunic (Retina)

Fibrous Tunic (Sclera)

→ Forms the cornea anteriorly; refracts light.

Vascular Tunic (Choroid)

→ Contains blood vessels and melanin to absorb light.

Ciliary Body and Suspensory Ligaments

→ Suspend and change shape of the lens.

Lens

→ Refracts (bends) light.

Iris

→ Smooth muscle that regulates light entering the eye.

Pupil →

Opening in the iris allowing light to enter the eye.

Retina (Nervous Tunic)

→ Contains photoreceptor cells (rods and cones); has pigmented layer with melanin.

Visible Light Range

→ 380–750 nm.

Rods

→ Photoreceptors for dim light and colorless vision; ~160 million; more sensitive than cones; detect shapes and movement; contain pigment rhodopsin.

Cones

→ Photoreceptors for color vision; ~7 million; concentrated in fovea centralis of macula lutea.

Optic Disk (Blind Spot)

→ Area of retina with no photoreceptor cells.

Fovea Centralis

→ Area of sharpest vision; high concentration of cones.

Cone Types and Peak Sensitivity

→ Blue (445 nm), Green (530 nm), Red (625 nm).

Visual Neuronal Pathway

→ Retina → Superior colliculus (mesencephalon, for visual reflexes) → Occipital lobe (visual cortex).

Olfaction

→ Sense of smell.

Olfactory Receptors

→ Chemoreceptors located in the upper nasal cavity.

Olfactory Epithelium Components

→ Receptor cells, supporting cells, and basal cells.

Olfactory Bulb and Tract

→ Transmit smell signals to the brain.

Olfactory Pathway

→ Odor molecules → Olfactory receptors → Olfactory bulb → Olfactory tract → Cerebral cortex and limbic system.

Gustation

→ Sense of taste.

Primary Taste Sensations

→ Sweet, sour, salty, bitter, umami.

Taste Receptors

→ Chemoreceptors located in taste buds on the tongue.

Taste Bud Structure

→ Taste (gustatory) cells, supporting cells, and basal cells.

Gustation Pathway to CNS

→ Taste receptors → Cranial nerves VII, IX, X → Medulla → Thalamus → Gustatory cortex.

Hearing

→ Sense of sound perception.

Outer Ear Components

→ Auricle (pinna), external auditory canal.

Middle Ear Components

→ Tympanic membrane (eardrum), ossicles (malleus, incus, stapes).

Inner Ear Components

→ Cochlea, semicircular canals, vestibule.

Cochlea

→ Spiral-shaped organ responsible for hearing; contains bony and membranous labyrinths.

Organ of Corti

→ Contains hair cells (mechanoreceptors) that transduce sound waves into nerve impulses.

Hearing Pathway to Brain

→ Sound waves → Tympanic membrane → Ossicles → Cochlea → Vestibulocochlear nerve (CN VIII) → Auditory cortex (temporal lobe).

Static Equilibrium

→ Detected in vestibule; concerned with head and body movement in one plane.

Dynamic Equilibrium

→ Detected by semicircular canals; senses rotation and motion in multiple planes.